This invention relates to a drive wheel suspension system for a motorcycle of the kind in which the drive wheel is driven by a chain cooperating with sprockets provided on the engine shaft and on the wheel shaft. In such motorcycles, it has been common practice to employ a swing arm for mounting the wheel, the swing arm having a pivot usually located in the region between the engine sprocket and the periphery of the driven wheel. With this configuration, the motion of the swing arm, which accompanies deflection of the wheel, results in change in the length of the path to be followed by the drive chain interconnecting the engine and wheel sprockets; and a chain slack take up device is commonly employed in order to maintain proper driving engagement of the chain with the sprockets.
It has also been known (see for example U.S. Pat. No. 4,058,181) to provide a parallelogram type of linkage for mounting the drive wheel, this linkage, in effect, comprising multiple swing arms which are interconnected and arranged to provide for pivotal motion of the drive wheel about the axis of the engine sprocket, thereby eliminating the change in the length of the chain path. Although this makes possible elimination of the chain slack take up device, the prior art arrangement has a number of disadvantages, notably the fact that the multiple swing arm elements required for the parallelogram linkage extensively increase the mass of the parts which must move with the wheel.
According to the present invention, provision is made for a parallelogram linkage adapted to be associated with the inner or pivoted end of the swing arm in a region between the periphery of the wheel and the axis of the engine sprocket, but arranged to provide an effective swing arm axis which is coincident with the axis of the engine sprocket. By locating the linkage parts in the region between the periphery of the wheel and the axis of the engine sprocket, the weight of the parts involved is concentrated relatively close to the effective axis of swing motion. This greatly reduces the overall weight of the parts and also reduces the inertia of the parts moving with the wheel and, consequently, improves the responsiveness of the wheel under various operating conditions including, for example, travel over rough terrain such as is commonly encountered in motocross racing.
The invention also contemplates association of the deflection resisting mechanism, such as a typical shock absorbing or shock strut device, with the parallelogram linkage parts located close to the effective swing motion axis, these parts being arranged to provide a reaction point on the frame of the motorcycle which is relatively close to the overall center of gravity of the motorcycle, thereby minimizing tendency for the action of the shock strut to cause the attitude of the motorcycle as a whole to change in consequence of substantial wheel deflection.